1. Field of the Invention
The present invention relates to an array substrate, and more particularly, to an array substrate including a thin film transistor having an improved stability in an electric property and a method of fabricating the array substrate.
2. Discussion of the Related Art
As the information age has progressed, display devices processing and displaying a large amount of information have been rapidly developed. Recently, flat panel display (FPD) devices such as a liquid crystal display (LCD) device, an organic light emitting diode (OLED) device, and an electrophoretic display device having the characteristics of thin profile, light-weight, and low power consumption have been developed to substitute for a cathode ray tube (CRT).
Among LCD devices, an active matrix type LCD device including an array substrate, where a thin film transistor that is a switching element controlling on and off of a voltage in each pixel region is formed, has been widely used because of its superiority in resolution and quality of displaying moving images. Further, the OLED device has a high brightness and a low driving voltage and is an emissive type that emits a light for itself. Accordingly, the OLED device has a high contrast ratio and a very thin profile. In addition, the OLED device has a good quality in displaying moving images because of a short response time of several micro seconds. The OLED device has no limitation in a viewing angle and is stably driven at a relatively low temperature. Since the OLED device is driven by a low DC voltage of about 5V to about 15V, it is easy to fabricate and design a driving circuit. The electrophoretic display device has been widely developed as a next-generation display device because of excellent contrast ratio, high brightness, low cost, and portability.
Each of the LCD device, the OLED device, and the electrophoretic display device includes an array substrate where a thin film transistor (TFT) is formed as a switching element to control the on and off states of a pixel region. Since a glass substrate is used for the array substrate of the display devices to tolerate the high temperature in the fabrication process, the display device has limitations in weight, profile, and flexibility. Accordingly, a flexible, thin display device including a substrate of a flexible material such as plastic instead of a glass substrate has been widely researched.
Since a flexible substrate such as a plastic substrate is inferior to a glass substrate in heat stability, the fabrication process for an array substrate using a flexible substrate is performed at a temperature lower than about 350° C. However, when a semiconductor material such as silicon is formed at a temperature lower than about 350° C. a semiconductor layer of the semiconductor material is degraded in electrical characteristics because of a low density. As a result, a thin film transistor including the semiconductor layer formed in a temperature lower than about 350° C. may not be used as a switching element.
To solve the above problems, an oxide semiconductor material that has an excellent semiconductor property even when it is formed in a temperature lower than about 350° C. has been developed. When a thin film transistor is fabricated by using an oxide semiconductor material, an oxide semiconductor material layer is stably formed at a temperature lower than about 350° C. In addition, since an ohmic contact layer is not required, the oxide semiconductor layer is not exposed to a dry etching gas, preventing deterioration of the properties of the thin film transistor.
In the array substrate including the oxide semiconductor layer, however, the property of the thin film transistor depends on the quality of a gate insulating layer that constitutes an interface with the oxide semiconductor layer. When the oxide semiconductor layer is formed through a sputtering method, the gate insulating layer may be formed of silicon oxide or silicon nitride through a chemical vapor deposition (CVD) method. When the oxide semiconductor layer is formed of a soluble oxide semiconductor material, the gate insulating layer may be formed of a soluble aluminum oxide material. For example, an aluminum oxide material layer may be formed on a substrate by coating a soluble aluminum oxide material and the substrate having the aluminum oxide material layer may be heated through a hardening step at a temperature of about 350° C.
When a positive bias temperature stress (PBTS) test as a reliability test is performed for the array substrate having a thin film transistor including the gate insulating layer of the soluble aluminum oxide material and the oxide semiconductor layer, a threshold voltage of the thin film transistor moves toward a negative voltage direction. In addition, when a negative bias temperature stress (NBTS) test is performed for the array substrate having the thin film transistor, the threshold voltage of the thin film transistor shifts along a positive voltage direction.
The negative and positive shifts of the threshold voltage in the PBTS and NBTS tests are caused by a relatively low density of the gate insulating layer of the thin film transistor. Since dehydration in the gate insulating layer of the soluble aluminum oxide material is not sufficiently performed by the hardening step of a temperature of about 350° C. a solvent in the gate insulating layer of the soluble aluminum oxide material is not sufficiently removed even after the hardening step. As a result, the gate insulating layer of the soluble aluminum oxide material has a relatively low density and there exists a large quantity of localized defects and mobile charges in the gate insulating layer. Accordingly, the property of the thin film transistor including the oxide semiconductor layer deteriorates under operation, as evidenced by shifts in the threshold voltage.